feat: add auto-select from preferences and type picker

- Add MediaTypeAsk (0) as the default, mapping old video_audio to Ask
- Show Video/Audio/Ask type picker when no default media type is set
- Auto-select matching format and skip picker when defaults are configured
- Add autoSelectVideoFormat / autoSelectAudioFormat helpers with closest-match
- Add buildFilteredFormatList for type-filtered format lists
- Add Media Type setting in settings UI (Ask/Video/Audio)
- Add --merge-output-format mp4 for video downloads via yt-dlp
- Fix formatLabelForDisplay to show Height fallback and Extension
- Add cookies volume mount to docker-compose.yml
- Document cookies setup and new features in README

internal/domain/types.go

@@ -6,13 +6,16 @@ import "os/exec"
 type MediaType int
 
 const (
-	MediaTypeAudio MediaType = iota + 1
+	MediaTypeAsk MediaType = iota
+	MediaTypeAudio
 	MediaTypeVideo
 	MediaTypeVideoAudio
 )
 
 func (m MediaType) String() string {
 	switch m {
+	case MediaTypeAsk:
+		return "ask"
 	case MediaTypeAudio:
 		return "audio"
 	case MediaTypeVideo: